Process for producing metal oxide fibers, textiles and shapes



United States Patent smear 13 Claims. or. zen-.5

ABSTRACT 0F THE Di'ElClLOSURE Metal oxide fibers, textiles and shapesare produced by heating a preformed, organic polymeric fiber, textile orshape impregnated with a metal compound. The temperature issuffic'iently high to carbonize and oxidize said organic polymericfiber, textile or shape without igniting same, and to convert the metalcompound to a metal oxide.

This application is a continuation-impart of copending application Ser.No. 320,843 filed Nov. 1, 1963, now abandoned, and Ser. No. 523,549filed Jan. 28, 1966, now abandoned, which latter application is in turna continuation-'in-part of copending applications Ser. No. 451,326 filedApr. 27, 1965,. now abandoned, Ser. No. 456,514 filed May 17, 1965, nowabandoned, and Ser. No. 522,380 filed I an. 24, 1966, now abandoned.

This invention relates to fibers, textiles and shapes composed of metaloxides which are substantially amorphous and to a process for producingthem.

The invention makes possible new kinds of metal oxide fibers, textilesand shapes having useful combinations of properties not feasible orpossible by other processes. For example, metal oxide fibers which havebeen produced in the past have all been subject to one or more of thedisadvantages of low softening and melting points, extremely shortlength, brittleness, or low tensile strength.

Metal oxide fibers presently made are all subject to the limitationsinherent in the process of drawing the fibers from the molten state andrapidly cooling before devitrification or crystallization of the fibers,particularly glass fibers, takes place. Metal oxide fibers produced bymelt-drawing are further limited to mixtures of oxides containingappreciable amounts of certain metal oxides capable of forming a glasswhen cooled from the molten states. Silica, SiO is the most widely usedmetal oxide, usually in amounts greater than 40 percent by Weight, informing glass fibers. Several other metal oxides can also be used toform fibers from the molten state, for example, B203, GCOZ, P205, AS203.

Metal oxide fibers produced heretofore further require the use of hightemperature melting furnaces and special crucibles and bushings fordrawing the fibers. The present invention provides a novel and greatlyimproved method for forming metal oxide fibers which does not requirethe use of high temperature melting equipment, nor is the use of glassforming oxides required as an essential constituent of the fiber. Also,there has heretofore been no completely satisfactory method forproducing metal oxide articles of predetermined irregular or complicatedshapes. Previous methods have involved machining or other shapingtechniques, or in the case of foams, the use of various blowing agents.These methods were either difiicult and complicated or were unable toprovide close control over the final shape of the article.

It is an object of the present invention to provide a novel class ofmetal oxide fibers which are not subject to the disadvantages andlimitations of metal oxide fibers previously known. Another object ofthe invention is to "ice provide fibers composed of a major amount ofone or more metal oxides, which fibers are characterized by theirsubstantially amorphous state, diameters in the range of 1 to 25microns, length-to-diameter ratios in excess of 400 and tensilestrengths in excess of 40,000 pounds per square Inch. A further objectof the invention is to provide a low-temperature process for producingfibers of metal oxides which have high melting points and which havehigh strength and flexibility. A still further object of the inventionis to provide a variety of textile forms, including staple fibers,continuous tow and yarns, woven fabrics, batting and felts, composed ofmetal oxide fibers.

It is another object of the invention to provide shaped metal oxidearticles, and a method for producing such articles from non-fibrousorganic materials. Still another object is to provide a variety offilms, tubes, cups and other shapes which are composed of metal oxides.

The fibers, textiles and shapes of the present invention are composed ofa total of at least percent by weight of oxides of one or more of themetals beryllium, magnesium, calcium, strontium, barium, scandium,yttrium, lanthanum cerium, and other rare earth elements, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, manganese, iron, c0- balt, nickel, copper, zinc, cadmium,aluminum, gallium, silicon, tin, lead, thorium, uranium and plutonium.Typical oxides of these metals are listed in Table I together with themelting point of the oxide or the temperature of decomposition in air(designated D) where the oxide does not melt. These metal oxides arecharacterized by decomposition or melting temperatures of at least 200C. above the minimum efiicient temperature (about 350 C.) employed inthe process of this invention. Other oxides, such as B O P 0 Na O, K 0,and the like, can be present in the metal oxide products of thisinvention but in amounts not greater than an aggregate of 20 percent byWeight.

TABLE I Decompo- Decompo- Metal sition (D) Metal sition (D) Oxide orMelting Oxide or Melting Temp, 0. Temp., C.

1 Includes other rare earth oxides of the formula M203, wherein M is arare earth metal.

The metal oxide fibers of this invention can exist in a wide variety oftextile forms, including staple fibers A inch to 3 inches or more inlength, continuous length, tow, yarn and roving, woven cloth, knits,braids, felts and the like. The non-fibrous shapes of this invention caninclude sheets, sponges, foams and the like.

The term amorphous as used herein, means that the metal oxide issubstantially micro-crystaline, that is, the crystallites are of suchsize that they are barely discernible by the conventional procedures ofX-ray difiraction. Hence the term micro-crystalline can be usedsynonymously with the term amorphous in describing the metal oxidematerials of the present invention. A

poorly discernable X-ray diffraction pattern for a crystalline materialis indicative of a crystallite size on the order of 1000 Angstrom unitsor less. It has been found that the metal oxide fibers of this inventioncan be prepared with crystallite sizes up to approximately 10,000Angstrom units, a size having a reasonably clearly defined X-raydefraction, before a significant loss in strength is experienced. Fibershaving crystallite sizes in the range below 1000 Angstrom units arepreferable, however, if maximum strength is to be retained.

The process for producing the metal oxide fibers, textile and shapes ofthis invention comprises the steps of (l) impregnating a preformedorganic polymeric material with one or more compounds (preferably saltsor hydrolysis products of salts) of metal elements which form the metaloxides of Table I and (2) heating the impregnated organic material undercontrolled conditions (which prevent ignition of the material) and atleast in part the presence of an oxidizing gas to (a) convert (pyrolyze)the organic material to predominantly carbon and thereafter remove thecarbon as a carbon-containing gas, and (b) oxidize the metalcompounds(s) to their respective metal oxide(s). There results a metaloxide fiber, textile or shape which has essentially the same physicalconfiguration as the original polymeric material.

The term preformed as used herein means that the organic polymericmaterial has been fabricated into a fibrous or non-fibrous shape priorto impregnation with the metal compounds.

The physical form and shape of the metal oxide product is essentiallythe same as and is determined by the physical form of the preformedorganic starting material, although considerable reduction in size takesplace. During conversion of impregnated organic fibers to the metaloxide fiber form, both the diameter and the length of the fiber shrinkto approximately 40 to 60 percent of the original dimensions. Similarshrinkage in all dimensions also takes place with the non-fibrousshapes.

Where a yarn composed of a multiplicity of continuouslength metal oxidefibers is desired, a continuous-filamine organic yarn is employed as thestarting material in the process of this invention. Similarly, where awoven fabric or felt composed of metal oxide fibers is desired, a wovenorganic fiber cloth or felt can be used as the starting material. Ofcourse, metal oxide woven textiles can be made using conventionaltextile equipment and techniques starting with metal oxide staple fibersor yarns made by the process of this invention.

Without being bound by same, the theory and mechanism of this processappears to be as follows: Microscopically, organic polymeric materials,such as cellulose, are composed of extremely small crystallites ofpolymer chains (micelles or microfibrils) held together in a matrix ofamorphous polymer. When the organic material is immersed in a solvent,such as water, aqueous solutions, or organic solvents it swells, thusopening the interstices between the crystallites. The amorphous regionsenlarge and the crystallite spacing increases. The dissolved metalcompound, such as a salt, enters the swollen amorphous regions, which isgenerally about 50 to 90 percent of the volume of the swollen organicmaterial, and becomes trapped in the amorphous regions between thecrystallites when the solvent is removed from the material.

The metal compounds do not crystallize upon drying of the organicmaterial, as would normally occur upon drying most solutions, since theyare effectively suspended and separated as islands, about 50 A. in sizein the case of cellulose, between the polymer crystallites.

The organic polymeric material can be impregnated with two or more metalcompounds from the same solvent solution, so that fibers, textiles orshapes containing more than one metal oxide can be prepared. In thefirst approximation, most metal compounds enter the interstices indirect proportion to their solution concentration, allowing readycontrol of the relative loadings of metal compounds in the organicmaterial. Due to the blocking action of the organic crystallites, themetal compounds cannot segregate from each other nor crystallize duringthe subsequent steps.

Any organic polymeric material can be employed as a starting material inthe process of this invention providing it is characterized by theabove-described structure of extremely small crystallites held togetherin a matrix of amorphous regions which enlarge and admit the metalcompounds on immersion in the solvent. Any class of materials which arecomposed of long-chain molecules held together by chemical cross-linkscan also be used, provided these materials are capable of swelling andabsorbing a solvent and provided the organic polymeric material does notmelt on heating. Any cellulosie material can be employed includingrayon, saponified cellulose acetate, cotton, wool and ramie, and thelike. Other suitable organic materials include the protein materials(such as wool and silk) and the man-made acrylics, polyesters, vinylsand polyurethanes. Certain organic materials, such as polyethylene andpolypropylene, are not suitable for practicing the instant processbecausethey cannot be swollen for imbibition of the metal compoundsand/or the materials melt and lose their structure during pyrolysis. Apreferred cellulosic material is rayon due to its structural uniformity,good imbibition characteristics and low impurity content.

Impregnation, or imbibition, of the organic matter can be carried out byseveral methods. Where the element which will appear in the final metaloxide article has salts which are highly soluble in water, theimpregnation step can be carried out by immersing the organic materialin a concentrated aqueous solution of such salt. For example, where analuminum oxide fiber is desired, an organic ifiber can be impregnated byimmersion in an aqueous solution of aluminum nitrate or aluminumchloride having concentrations in the range 2.0 to 3.0 moles of salt perliter. For salts which hydrolyze (acid reaction) when dissolved inwater, the acidity of the impregnating solution is preferably notgreater than 1.0 molar (in hydrogen ion) in order to prevent degradationof the organic material durin immersion. The acid may be neutralizedwith ammonia, if desired.

In order to obtain adequate strength in the final metal oxide product,cellulosic materials are imbibed with the metal compounds to the extentof at least one-quarter mole and preferably 1.0 to 2.0 moles of themetal compound(s) in each base mole, of cellulose. The term base mole asused herein refers to the molecular weight of a glycosidic unit of thecellulose chain (molecular weight of 162). With non-cellulosicmaterials, the degree of imbibition should be at least 0.1 andpreferably 0.5 to 1.0 gram-equivalent metal ion in the metal compoundimbibing solution per gram of organic material. With lowerconcentrations of metal compound(s), insufiicient metal salt isavailable in the relic fiber, textile or shape for a strong article andthe process becomes less efiicient in terms of product yield per unitweight organic starting material.

Pre-swelling of cellulosic organic materials in water prior to immersionin concentrated imbibing solutions is preferably employed to increaseboth the rate and extent of salt imbibition. For acrylic and polyestermaterials, aromatic alcohols are suitable swelling agents, and theketones are useful in swelling vinyl and polyurethane materials for thesame purpose.

Water is the preferred solvent for metal compoundimbibing of cellulosicmaterials. ()ther solvents such as alcohols do not afford as efficientswelling nor solubility of the selected metal compound for a high degreeof imbibing. For vinyl and polyurethane materials, esters and retonesare appropriate solvents, as for example normal butyl acetate or methylethyl ketone. For acrylic and polyester materials, suitable solvents forthe metal compound imbibition include aromatic alcohols and amines suchas aniline, nitro-phenol, meta-cresol and paraphenylphenol.

Immersion times at normal room temperatures (21- 23 C.) required to giveadequate impregnation vary from a few minutes to several days dependingon the salts(s) employed and the type of organic material employed. Forexample, at 21 C. water-swollen regular viscose rayon imbibes 0.9 moleUO Cl per base mole rayon from 3.3 molar solution in 30 minutes.Imbibition of AlCl from concentrated solutions at 21 C. takes place moreslowly. For example, under the same conditions as described above for UOCl water-swollen regular viscose rayon imbibes AlCl to the extent of 0.4mole per base mole rayon 30 minutes. After 3 days immersion, AlCl isabsorbed in rayon to the extent of 0.7 mole per base mole rayon.Immersion times greater than about 3 days in concentrated salt solutionsis undesirable for cellulosic material since the material may degrade,resulting in a decrease in the amount of salt absorbed, and in the caseof fibers, causing the fibers to bond to each other.

To further illustrate the impregnation step, viscose rayon rapidlyswells and absorbs ZnCl to a large extent in concentrated solutions.Within 15 minutes water-swollen viscose rayon absorbs 3.4 moles of ZnClper base mole of rayon from a 6.8 molar solution at 21 C. However, therayon impregnated by this treatment swells to essentially a gel andbecomes tacky. The rayon in this swollen state is too weak to be handledand the fibers cannot be separated from each other. The preferredprocess for impregnating rayon with ZnCl so that the salt-loaded rayonis not degraded and fibers are not bonded together, is immersion ofwater-swollen rayon in 3.6 to 4.0 molar solution for 1 to 3 hours. Bythis treatment, viscose rayon absorbs 0.6 to 1.0 mole of ZnCl per basemole of rayon.

When it is desired to increase the rate of imbibition of the metalcompounds in the organic materials to shorten immersion time, the metalcompound solution may be heated to as high as 100 C. For example, forsalts which are taken up by cellulosic fibers at a slow rate, such asAlCl- AMNOQ ZIOCl-g, and ThCl the immersion time can be shortened byraising the temperature of the salt solution to 50 C. to 65 C. Careshould be exercised, however, in using elevated temperatures since manysalts will grossly degrade the organic material at high temperatures.

An alternate method of loading the organic polymeric material is toemploy compounds which hydrolyze or react with water to form metal oxideproducts which are essentially insoluble in water. This chemicalproperty is utilized to effect impregnaion of organic materials withthese metal oxides as described below. Suitable hydrolyzable and/orwater reactive compounds include the following: (1) VOCI VCI VCL; toyield V (2) NbOCl NbCl NbBr to yield NbgGf; hydrate; (3) TaCl TaBr toyield "21 0 hydrate; (4) MoCl Mo O Cl to yield MoO hydrate; (5) WCI WCIto yield W0 hydrate (tungstic acid); and (6) SiCL; or silanes such astrimethylsilane to yield SiO Th above named metal halides or oxyhalidesare dissolved in an organic liquid immiscible with water, such as carbontetrachloride, chloroform, carbon disulfied, ethyl ether, or benzene, tothe extent of 5 to 50 g. of metal halide or oxyhalide per 100 ml. oforganic liquid. The rayon or other cellulosic or organic material isexposed to air having a relative humidity between 50 and 90 percent, inorder for the material to swell by absorbing between 5 and 30 percent byweight of water. While still swollen and containing the absorbed water,the organic material is contacted with the metal halide or oxyhalide byimmersion in liquid or gaseous halide or oxyhalide or in organic solventsolutions of halide or oxyhalide. As the metal halide or oxyhalidepenetrates the moist material, reacts with the water, and an oxideprecipitate forms directly in the organic material structure. Thishydrolysis reaction is normally complete in 20 to 30 minutes.

The extent or amount of metal deposited within the organic material isdirectly a function of the amount of water absorbed in the material.Typical hydrolysis reactions are the following:

The amount of water absorbed in the organic material is readilycontrolled by exposing the material to air containing the desired amountof moisture. For maximum water absorption the material can be immerseddirectly in liquid water. For example, the amount of water absorbed intextile-grade, viscose rayon fibers in equilibrium with moisture in airand liquid water at F. is shown below:

Moisture content, percent Relative humidity at 75 F.: of dry fiberweight 70 14 17 23 30 (immersed in water) 80-110 Some hydro-lyzablemetal compounds are liqiud at normal conditions and the water-ladenorganic material can be immersed directly in the metal compound to causethe hydrolysis product to be formed in the mate-rial. Examples of liquidmetal halides are SiCl TiCl VOCl and VCI However, many of the hydrolysisreactions proceed very rapidly with the evolution of heat. The resultingsevere conditions may degrade or break up the organic material; in thisevent the metal compound is preferably diluted with a non-reactive,miscible liquid to avoid such conditions. Many non-polar organicliquids, such as benzene, toluene, hexane, carbon tetrachloride,chloroform, are suitable non-reactive liquids. These organic liquids,when used as diluents for the metal compounds, slow the rates ofhydrolysis and help to dissipate the heat of reaction. Unreacted metalcompound liquid (as well as any diluent) may be removed from between thefibers by evaporation, since they have high vapor pressures.

Other metal compounds which can be incorporated in polymeric organicmaterials by hydrolysis reaction but which are not normally liquids arebest utilized when dissolved in a non-reactive liquid which isimmiscible with water. Such metal compounds, for example, include TaClNbCl ZrCl UCl Suitable solvents are bromoform, carbon tetrachloride,diethyl ether, and nitrobenzene.

Following imbibition with metal compounds(s) from a solvent solution, itis necessary to remove excess solution from between the organic fibersbefore they dry in order to avoid bonding together of fibers by cakedsalt, or from the surface of non-fibrous shapes in order to preventaccumulation of caked salt on the surfaces of the shape. Allowing excessunim'bibed metal or hydrolysis product to remain results in reducedstrength and increased brittleness in the final metal oxide product.Blotting thoroughly with absorbent paper or cloth using moderatepressure is useful for removing excess solution from the organicmaterial. In addition, washing, high velocity gas streams, vacuumfiltration and centrifugation have proven to be effective methods forremoving excess impregnating solution. For solutions, such as 3.0 molarAlCl having viscosities greater than about 10 centipoises, raising thetemper'ature of the organic material to 5060 C., aids in removing excesssolution.

The impregnated organic material is then thoroughly dried by anyconvenient means, such as air drying or heating in a stream of warm gas.It is desirable to dry the impregnated fibers rapidly (in about one houror less) drolysis of the metal halide or oxyhalide from organicsolution, a preferred method is to impregnate first with the hydrolysisproduct and then with the water soluble salt.

In the next principal step in the process of this invention (theconversion of the impregnated organic material to metal oxide) theimpregnated organic material is heated under controlled conditions for atime sufficient to decompose the organic structure and form acarbonaceous relic shape containing the metal compound in finelydispersed form, and concurrently and/or subsequently to eliminate thecarbon and convert the metal compound to metal oxide.

The controlled conditions must be such as to avoid ignition of theorganic material. For the impregnated organic materials used in theprocess of this invention, ignition generally takes the form of anuncontrolled temperature increase within the material rather thancombustion accompanied by flame. An uncontrolled temperature increase isa rapid increase which deviates sharply from the heating pattern of theimpregnated organic material and its environment. When pyrolysisconditions are properly controlled, the temperature increase in theimpregnated organic material follows closely the temperature of itssurroundings (atmosphere, furnace wall, and the like) even though theexact temperature of the organic material may fluctuate to temperaturesboth above and below the nominal temperature of the environment. If theorganic material ignites or burns instead of carbonizes, the metalcompound temperature rises excessively due to its contiguous relation tothe organic structure. Under such circumstances it is impossible tocontrol the temperature, and the melting point of intermediate metalcompounds formed may be exceeded or excessive crystallization and graingrowth can occur. Also the metal compound may become suspended in thepyrolysis product vapors, and thus lost from the environment andunavailable to form the desired relic. When ignition is avoided theproducts have smoother surfaces and are stronger due to a more orderlyconsolidation of the metal compound particles.

In practice, a convenient way of determining whether or not ignition hastaken place during the heating steps is to observe the degree ofshrinkage or consolidation of the starting organic material. Whereignition has not taken place, the impregnated organic material undergoessubstantial shrinkage along its longest dimension, generally in theorder of 40-60 percent. (Shrinkage from 10 cm. long to cm. long is 50percent shrinkage.) The final product is strong and microcrystalline,and in the case of fibrous products, highly flexible. On the other hand,where undesirable ignition takes place during the heating steps, thedegree of shrinkage is considerably less, and the resulting producttends to be crystalline rather than microcrystalline and is brittle andof low strength. It ignition takes place toward the end of thecarbonization-oxidation step, the degree of shrinkage may still besubstantial but the physical properties of the product will be lessdesirable. In general, the degree of shrinkage is inversely proportionalto the loading of metal compound into the organic material. It has beenfound desirable to adjust process conditions to obtain maximum shrinkagefor the particular metal compound loading in the impregnated organicpolymeric material.

In practice, it has been found that ignition can be avoided by use ofcontrolled reaction conditions, particularly conditions which avoidsharp changes in temperature, atmosphere composition and the like. Sharpchanges in conditions tend to precipitate the uncontrolled temperatureincreases within the impregnated organic material which constituteignition as hereinabove defined.

As an example of such controlled condition, cellulosic fibersimpregnated with metal salts are heated to a temperature between about350 C. and 900 C. at a rate of not more than C. per hour in anatmosphere containing between 5 and about 25 volume percent oxidizinggas. (It is understood, of course, that where the metal oxide product isto contain one of the oxides of Table I which has a decomposition ormelting point below 900 C. the impregnated organic material is notheated above such decomposition or melting temperature.) By the time theimpregnated fiber has been heated to 350 C. or above, under theabove-described conditions, a major portion of the cellulosic fiber willhave been pyrolyzed to carbon (carbonized) and the carbon removed as acarbon containing gas through reaction with the oxidizing gas(Volatilized), and a major portion of the metal element in theimpregnated fiber will have been oxidized to the metal oxide form. Theimpregnated fiber should, however, be maintained at a temperaturebetween 350 C. and 900 C. in an oxidizing gas-containing atmosphereuntil substantially all of the impregnated fibers have been carbonizedand volatilized and substantially all of the metal has been oxidized tothe metal oxide form.

After the first slow heating to a temperature above 350 C. the volumepercent of oxidizing gas need not be maintained at 25 volume percent orbelow, although there is generally no signficant advantage in employingan atmosphere containing greater than 25 volume percent oxidizing gas.The preferred oxidizing gas is oxygen, although other oxidizing gasessuch as nitrogen dioxide and sulphur trioxide can be used if desired.The balance of the gaseous atmosphere comprises gases which arechemically nonreactive at temperatures up to 900 C. and above. Typicalnon-reactive gases include nitrogen, helium, argon, neon, and the like.

A preferred embodiment of this invention for producing fibrous metaloxide products comprises 1) impregnating a cellulosic fiber as describedhereinabove with compounds of metals of Table I whose oxides havemelting or decomposition temperatures above 800 C., and (2) heating thefiber to a temperature between about 400 C. and 800 C. at a rate between10 C. per hour and 100 C. per hour in an atmosphere containing from 5 to25 volume per cent oxygen, and thereafter maintaining the fibers at atemperature between 400 and 800 C. in an oxygen-containing atmosphereuntil substantially all of the fibers have been carbonized andvolatilized and substantially all of the metal has been oxidized to themetal oxide form.

In this process of this invention, the important factor is that theprocess variables be controlled to avoid ignition and/ or burning of theorganic portion of the impregnated organic material. Control of processvariables to avoid ignition can be carried out in many ways; forexample, by temperature regulation, by limiting the amount of oxidizingagent available to the impregnated material, or by the use of vacuum orinert atmosphere. It must be remembered that a certain amount of oxygenforms a part of the chemical structure of many organic materials, forexample cellulose, so that a certain amount of oxidizing agent isavailable within the impregnated material itself. Here the use ofsmaller samples of impregnated material or relatively slow heatingrates, particularly to temperatures up to about 350 C., can help toavoid undesirable ignition. It has been found, however, that with manysystems, particularly impregnated cellulosic fibers from which excesssolvent has been removed and which have been carefully dried, that rapidheating in vacuum or inert atmosphere is possible without causingignition.

In the step in the process of this invention in which the metal compoundis oxidized to metal oxide, it is necessary to supply some type ofoxidizing agent. As indicated above, this is most conveniently done byheating the carbonized (and in many instances partially oxidized)material in an atmosphere containing oxygen or other oxidizing gas. Itis of course necessary to avoid ignition during the oxidation step also.This can again be achieved by careful control of temperature, bylimiting the amount of oxidizing gas supplied, or by employingrelatively unreactive oxidizing agents. An example of this lattertechnique is to employ water vapor at temperatures above about 700 C. asthe oxidizing agent. Particularly good results have been achieved byheating the carbonized material in an inert gas containing a few percentwater vapor.

Control of conditions to avoid ignition is generally easier with fibersand fibrous materials such as textiles than with other non-fibrousshapes, such as organic forms or sponges, which have been impregnatedwith metal compounds. For such non-fibrous shapes it is generallypreferred to use non-oxidizing atmospheres in the initial portion of thecarbonization-oxidation process and to maintain the rate of heating atless than 50 C. per hour. An oxidizing agent can thereafter be added tothe atmosphere after a substantial portion of the carbonization step iscompleted. It is also possible to carry out the entirecarbonization-oxidation treatment of non-fibrous shapes by heating atrates between 10 C. and 50 C. per hour in atmospheres containing from to25 volume percent oxygen.

For both fibrous and non-fibrous materials it is seldom necessary toemploy temperatures above 1000 C. The last traces of carbon can beremoved and the last traces of metal compound oxidized by employingtemperatures of up to 1000 C. and oxygen concentrations of about 20volume percent.

The exact choice of reaction conditions of course depends on the shapeand chemical composition of the starting organic material, and on themetal compound or metal compounds employed in the impregnation step.Illustrative of the permissible variation in reaction conditions (solong as ignition is avoided) are the following three methods forproducing zirconium oxide fibers from imprenated cellulosic fibers.

In all three methods, cellulosic fibers are impregnated by immersingthem in an aqueous solution of zirconyl chloride. Excess solution isthen carefully removed and the impregnated fibers are dried. In thefirst method the carbonization-oxidation step is carried out by heatingthe fibers in air at a uniform rate from room temperature up to 400 C.over a 24-hour period. The fibers are then heated in air at temperaturesbetween 400 C. and 600 C., preferably closer to 600 C., for anadditional period of up to twenty-four hours. This procedure isillustrative of ignition control by slow oxidation over a long period oftime.

In the second method the carbonization-oxidation step is carried out byheating the impregnated fibers in an argon atmosphere from roomtemperature to temperatures between 600 C. and 1000 C. in less than onehour. The oxidation is then carried out by maintaining the fibers atabout 800 C. in an atmosphere of argon containing at least 5 volumepercent water vapor for a period of one to ten hours. This procedure isillustrative of ignition control by rapid pyrolysis in a non-reactiveatmosphere followed by rapid high temperature oxidation using oxidizingagents of relatively low reactivity.

In the third method the impregnated fibers are rapidly heated, often inless than five minutes, to about 400 C. in an atmosphere containing lessthan approximately volume percent oxygen. The oxygen partial pressure isthen gradually increased at constant temperature until the carbonaceousmaterial from the cellulose and the metal compound have been oxidized,generally in a period of /2 to 3 hours. This procedure is illustrativeof avoiding ignition by direct control of the oxygen partial pressure inthe environment of the fiber.

The various methods for carrying out the process of this inventionwithout ignition of the organic material, as illustrated above in thecase of production of zirconium oxide fibers, apply generally to theproduction of any of the fibrous products of this invention; that is,impregnated fibrous materials can be heated in non-oxidizing atmosphere(conveniently provided by the use of vacuum or an inert gas) totemperatures between about 700 C. and l000 C. in a period of less thanone hour, followed by additional heating in this same temperature rangein an atmosphere containing at least five volume percent water vapor fora period suflicient to complete the oxidation step, generally betweenone and 10 hours.

Similarly, the impregnated fibrous material can be rapidly heated tobetween about 350 C. and 600 C. in an atmosphere containing less thanabout 10 volume percent oxygen, and then maintained in this sametemperature range while the oxygen partial pressure in the atmospheresurrounding the fibrous material is gradually increased to 20 volumepercent or more over a period of a few minutes to a few hours dependingon the particular system involved, the time being long enough tocomplete the oxidation step. Even though the oxygen partial pressure canbe increased over short periods of time, abrupt increases should beavoided.

The process of this invention can also be practiced on a continuousbasis. For example, continuous metal oxide filaments can be produced byproviding a spool of cellulosic fiber, yarn or the like, drawing thefilament or fiber first through an impregnating solution, then through ablotting and drying operation, and finally through one or more furnaceswhich provide carbonization and/ or oxidation under the necessarycontrolled conditions to avoid ignition. The metal oxide fiber emergingfrom the final furnacing under operation can be continuously wound onanother spool, thus providing an efficient continuous method forcarrying out the process of the invention.

The carbonization of the impregnated organic materials and the removalof such carbon by oxidation are not necessarily separate and distinctsteps. When heating of the impregnated material is first begun,pyrolysis to carbon is the predominant chemical reaction, and thecarbonized material comprises predominantly carbon but also can containsmall amounts of residual organic material, including hydrogen andoxygen. However, some slight oxidation of the carbon formed and of themetal present in the impregnated material can take place, particularlywhen the atmosphere employed contains an oxidizing gas. As heatingcontinues and substantially all of the organic material is converted tocarbon, oxidation by the oxidizing atmosphere of the carbon and of themetal with which the material had been impregnated becomes thepredominant reaction. In the later stages of the heating process,oxidation of the last few metal atoms and of the last few carbon atomsis the only significant chemical reaction. In many instances, theaddition of small amounts of water vapor to the atmosphere aids inremoving the last traces of carbon.

Microscopic voids are produced in the material as carbon is oxidized.Maximum densification of the material is achieved by limiting theconcentration of oxidizing gas and the temperature of the materialduring oxidation. This oxidation step normally requires between one and48 hours to complete, depending on the metal oxide or metal oxidemixture. Many metal oxides, such as uranium oxide, zirconium oxide, ironoxide, copper oxide, chromium oxide and vanadium oxide, enhance the rateof carbon oxidation and relatively lower temperatures are normally usedwhen these metal oxides are present in the material. Oxidation of thecarbon at a faster rate (promoted by either increased oxidizing gasconcentration or relatively higher temperature in early stages of thisoxidation step) produces a less dense and weaker metal oxide product dueto the voids remaining in the structure. The relatively highertemperatures may also initiate crystallite formation in the materialwhich is detrimental to th metal oxide product in its formatory stages.Densification of the metal oxide relic is Observed as shrinkage alongall dimensions during oxidation. For fibers, the lengthto-diameterratio, as well as the eon-retry of the fiber cross-section, remainsessentially the same as the starting organic fibers during conversion.

In a particularly preferred embodiment of the present invention, where atextile yarn containing a multiplicity of metal oxide fibers is desired,an imprea llulosic yarn is kept under tension during the caroomzationand oxidation steps. Tensions in the 1G to 40 gram have provensatisfactory for keeping 3300 denier/Lat) filament rayon yarns straightduring the carbonization and oxidation steps.

Although the metal oxide products as produced by the process of thisinvention are substantially amorphous (microcrystalline), theirapplication is not limit d to conditions and environments wherein themetal oxide products remain entirely in the microcrystalline state. Thedesirable mechanical properties of the metal oxide products inventionare retained to a large degree even after some crystallization of themetal oxides has occurred. The mechanical properties of the products areseriously impaired only after the size of the crystallites is such thatcrystalline grains can be detected by conventional optical microscopictechniques, that is after the microcrystalline structure is replaced byrelatively large rystalline areas. In the case of metal oxide fibers,mechanical properties become impaired when the size of the crystallitesexceeds approximately one-tenth of the diameter of the fiber.

The metal oxide products or" this invention which are most referred arethose which are composed almost entirely of one or more of the metaloxides of Table I hereinabove, and which contain only trace ofinsignificant amounts of other metal oxides or impurities.

The metal oxides listed in Table l are the oxides usually formed when anorganic material impregnated with compounds of one or more metals ispyrolyzed and oxidized according to the process of this invention andrepresent the highest normal oxidation state of the metal. However thepresent invention includes metal oxide fibers, textiles and shapescomprising a total of at least 80 percent by weight of lower metaloxides (including oxides of non-integral valence states) of the elementsof Table I, for example FeO, Cr() and U The lower metal oxides can beobtained by partial reduction of the higher oxides with hydrogen attemperatures above 400 C., or can be obtained as intermediate oxidationstates during the oxidation step in the process of this invention.

A particularly important class of metal oxide products are thosecomprising a total of at least 80 Wei ht percent of one or more of themetal ox des having melting points above 1728 C. (the melting point ofsilica), and in par- "cular products composed principally of one or moreof the oxides A1 0 EeO, CaO, CcO MgO, TiO T210 and Z which possessexceptional chemical inertness and strength at high temperatures. Thesemetal oxides in dense, sintered forms comprise an important class ofcommercial refractory oxides. Fibers of these refractory metal oxidesmake good heat shields and ablation reinforcement materials, and can beemployed for reinforcing plastics for use at relatively lowtemperatures, and metal and porcelains and other ceramic bodies for useat high tem peratures. These refractory metal oxide fibers makeexcellent filters for corrosive gases and liquids at elevatedtemperatures. Applications include filtering molten metals, moltensalts, superheated air and furnace exhaust gases. In addition tofiltering applications, the metal oxide products of this invention arealso useful as structural elements and as thermal insulation elementsfor use at high temperatures and in corrosive atmospheres, and theproducts of this invention which contain cerium oxide, thorium oxide,and zirconium oxide are particularly ad- 12 vantageous for use incontact with corrosive or alkaline liquids such as battery e ctrclyZirconium oxide fibrous materials are especially us ful because of theirlow thermal conductivity and because of their extreme high temperaturestability even in alkaline environments.

Fabrication of ceramic blocks, linings, crucibles and more complexshapes are often difi..ult because of the problems attendant to fusingand sintering them into a cohesive mass. Refractory bodies frequentlysuffer selfdestruction if temperatures are varied abruptly due to theirpoor resistance to thermal shock. Startl g with these refractory fibersin the form of loose fibers, yarn, paper or various forms of woventextiles, shaping and sintcring to cohesive, complex shapes can beachieved readily. Thin-walled. tubes ma. be made by this method andapplications include furnace liners, pyrometer protection tubes and forliquid metals.

The metal oxides of Table l which have relatively low melting ordecomposition temperatures, for example, the oxides of vanadimolybdenum, tungsten, manganese, cobalt, nickel, co r, zinc, cadmium andlead are known to be useful as catalysts in a variety of processes. Thefiber form of these oxides, as produced by the process of thisinvention, possess the same catalytic properties and catalytic uses asthe conventional non-fiber form of these oxides. A particularly usefulform of catalyst, for exampie, is a fiber of this invention whichcomprises from 80 to 98 percent of one of the refractory me I oxides andfrom to 2 percent of one of the lower melting point catalytically activemetal oxides.

Micron-size fibers containing uranium oxide or plutonium oxide areuseful for fuel elements in nuclear reactors, especially those designedfor radiation-chemical processing by the use of the kinetic energy offission recoil particles. The fibers can be composed of only uraniumoxide or plutonium oxide, or as a mixture with other metal oxides ofTable I, such as BeO, A1 0 ZrO and T110 For use as fuel elements inother types of nuclear reactors, for research and testing purposes,vehicle propulsion, or electrical power generation, it may be desirableto coat these fibers with an impervious material to prevent the escapeof radioactive fission products into the reactor coolant stream. For useat high temperatures or in nonoxidizing atmospheres, uranium dioxide, U0is the preferred form and is obtained by reducing the uranium trioxide,U0 with hydrogen gas at 500-600 C. A preferred fiber of this inventionfor use as a nuclear fuel element under oxidizing conditions is onewhich comprises from 20 to weight percent uranium trioxide and thebalance to 40 weight percent) aluminum or Zirconium oxide.

Metal oxide fibers of this invention containing uranium dioxide aregenerally useful as nuclear fuel elements, and fibers comprising amixture of uranium dioxide and thorium dioxide are particularly usefulas fuel elemnts in breeder reactors.

The metal oxide shapes of this invention have a wide variety of uses.The metal oxide films of this invention, which are highly uniform inthickness and which can be as thin as 10 microns, can be used as thindielectric or heat insulating films or sheets (for example aluminumoxide films and other ceramic oxide films). The metal oxide shapes ofthis invention other than films can be used as light weight structuralmembers, as heat and/ or electrical insulators, as battery separators,and the like. Metal oxide shapes prepared from organic foams or spongesare also useful as filters. For use as filters it is preferred that themetal oxide shapes of this invention be prepared from cellulosic foamsor sponges characterized by open porosity, uniform pore size and lowdensity.

The following examples further illustrate the process and metal oxideproducts of this invention. Throughout the present specification,including the illustrative examples, the temperatures given are furnacetemperatures. The actual temperature of the structure undergoingprocessing by the method of this invention, as discussed 13 hereinabove,may differ somewhat from the furnace temperature.

EXAMPLE 1 A 50 ft. length of rayon tire yarn (3300 denier/1440filaments/1 ply) weighting 5.6 grams was preswollen by immersion inwater for 1.5 hours. After centrifuging out excess water, the yarncontained 0.70 gm. water/ gm. rayon and was immresed in 2.8 molaraluminum chloride .(aqueous) solution held at 22 C. for a period of 22hours. After centrifuging excess solution from the yarn and allowing itto dry, the yarn contained 0.79 gm. of aluminum chloride salt/gm. rayonand retained its original luster and flexibility. A two-foot length ofthe yarn loaded with aluminum chloride was converted to alumina yarn byheating in air to 400 C. at a rate of 100 C./hour and maintaining thetemperature at 400 C. for two additional hours. To remove final tracesof carbon from the alumina yarn, it was heated at 800 C. in air for aperiod of six hours. The yarn was maintained under a tension of gramsduring conversion to keep the filaments in the yarn straight.

The alumina yarn had a high luster and flexibility much like thestarting rayon yarn. The converted yarn had a denier of 1260 and hadshrunk to 46 percent of its original length. In four measurements oftensile strength using an Instron testing machine, two-inch lengths ofthe alumina yarn supported 1.6, 1.6, 1.6 and 1.9 pounds tension beforebreaking. Individual filaments were next taken from the alumina yarn andtheir tensile strengths determined using a modified analytical chaintype balance for applying a known amount of tension to the fibers. Inthree measurements, the filaments supported 2.0, 1.6 and 1.3 gramsbefore breaking. Tensile strengths of the individual alumina filamentsare calculated to be between 85,000 130,000 pounds per square inch.

EXAMPLE 2 This example illustrates the impregnation and conversion of awoven cloth into a woven metal oxide (alumina) cloth. A one-square footpiece of cloth weighing 41.4 gm., made up of textile yarn (3300 denier/1100 filaments/ 3 ply), and having 17 yarns/inch warp and 8 /2 yarns/inch fill, was preswelled by immersion in water for one hour. Afterthorough blotting, the cloth contained 0.81 gram water per gm. cloth.The water-swollen cloth was immersed in aqueous aluminum chloridesolution for a period of 65 hours. Solution concentration at the end ofthis period was 2.5 molar AlCl After centrifuging excess solution fromthe cloth, it was dried rapidly in recirculating air heated to 50 C. Thedried cloth contained 0.69 gm. salt/ gm. of rayon and was converted toalumina by heating in air gradually to 400 C. in a period of 48 hours.Traces of carbon were removed from the cloth by heating in air at 800 C.for five hours.

The white alumina cloth so produced had a high luster and was veryflexible. The alumina cloth weighed 7.1 grams and had shrnuk to an areaof 0.16 square foot. A pull of 3.6 pounds per inch of width was requiredto tear the cloth. Spectographic analysis of the alumina cloth showed itto contain 99.5 percent A1 0 The major trace impurity was Zn, which wascontained in the starting rayon cloth and could have been eliminated bywashing the rayon with dilute hydrochloric acid. X-ray powderdiffraction analysis showed the cloth to be essentially amorphousalumina. A trace of poorly crystallized gammaalumina was present asindicated by a broad diffraction band at a 20 diffraction angle of 44-46degrees.

EXAMPLE 3 A piece of rayon felt, weighing ounces/yd was impregnated withaluminum chloride salt and converted to alumina felt by the methoddescribed in Example 2. The alumina fel-t weighed 19 ounces/yd. and hada bulk density of 7.9 pounds/ftfi The felt possessed a high de- 1 4 greeof flexibility and -a breaking strength of two pounds/ inch of width.

EXAMPLE 4 A one-square foot piece of rayon cloth weighing 56.2 gms.,made up of high-tenacity rayon yarn in a basket weave and having a yarncount of 19 yarns/inch in both warp and fill directions, was preswollenin water and immersed in a 2.86 molar zirconyl chloride aqueous solutionat 22 C. for a period of 46 hours. The solution concentration at the endof the immersion period was 2.55 molar. The cloth was centrifuged anddried rapidly in recirculating air heated at 50 C. The dried clothcontained 0.96 grams of zirconyl chloride salt/ gram rayon. Thesalt-loaded rayon cloth was converted to zirconium oxide cloth byheating in air gradually to 500 C. in a period of 30 hours (about 16 C.per hour). The cloth was maintained at 500 C. in air for six additionalhours to remove remaining traces of carbon. The tan-colored Zirconiumoxide cloth weighed 14.2 grams and had a yarn count of 48 yarns/inch.The cloth was flexible and had a breaking strength of six pounds perinch of width. X-ray diffraction pattern showed the zirconia cloth to bepredominately amorphous but containing trace amounts of poorlycrystallized tetragonal zirconia.

EXAMPLE 5 Forty-five grams of 1.5 denier regular viscose fibers wereimpregnated with a mixture of uranyl chloride and aluminum chloride byimmersion of the preswollen fibers in a single aqueous solution 0.4molar in uranyl chloride and 2.8 molar in aluminum chloride for a periodof 48 hours. After centrifugation to remove excess solution and dryingthe fibers in a stream of warm air, the loaded rayon fibers wereconverted to the oxide form by heating in air at a rate of 50 C./hr. to400 C., and were freed of carbon by maintaining them in air at 400 C.for 4 hours. The product metal oxide fibers were composed of 56.5% byweight U0 and 43.5% by weight Al O The amber colored mixed oxide fibershad a high degree of luster and had tensile strengths Very nearly thesame as the pure alumina fibers described in Example 1. The mixed oxidefibers had no crystal structure, as shown by X-ray diffraction.

EXAMPLE 6 Four aqueous solutions were prepared which contained salts inthe following concentrations: (a) 2.3 molar in AlCl and 0.34 molar inNiCl (b) 2.3 molar in AiCl and 0.32 molar in CrCl (c) 2.3 molar in AlCland 0.27 molar in FeCl and (d) 2.3 molar in AlCl and 0.33 molar in CuClFour separate samples of 1.5 denier regular viscose rayon fibers werepreswollen and immersed in the four solutions, respectively, for 48hours. Excess solution was removed by centrifugation and the fibers weredried in warm air. The impregnated fibers were then heated to 400 C. inair at a rate of 50 C. per hour and were then maintained in air at 400C. for four additional hours. The four metal oxide fibers which resultedcomprised, respectively, 13.0 weight percent nickel oxide, 11.7 weightpercent chromium oxide, 15.9 percent iron oxide, and 13.2 percent copperoxide, with the balance in each case aluminum oxide. All four of themixed oxide fibers had high tensile strength and were completelyamorphous.

EXAMr LE 7 A knitted rayon sock impregnated with a thorium compound, anarticle of commerce for use as an incandescent mantle in a Coleman gaslantern, was converted to the oxide by heating in air to 400 C. in aperiod of 7 hours and holding at 400 C. in an oxygen stream for anadditional 24 hours. After this treatment the rayon was completelydecomposed and the carbon volatilized from the knitted sock. Theresulting sock was composed of white, lustrous thoria fibers of anamorphous structure. The

sock was extremely flexible, similar to the starting rayon knitted sock.When viewed under the microscope at a magnification of 50, the thoriafibers appeared transparent to light (similar to window glass).

A second, and identical, knitted rayon sock impregnated with a thoriumcompound was burned off to the oxide form rapidly with a gas flame as isnormally done for use as an incandescent gas mantle. The resultingthoria sock contained white fibers with a dull luster. The fibers alsowere very brittle and had little strength. X-ray diffraction patternshowed the thoria to be fairly well crystallized. The brittleness andlow strength of thoria fibers made by burning off the rayon isattributed to crystallization and a large number of voids remaining inthe fiber structure. In contrast, useful properties, such as highstrength and flexibility, are developed by slow conversion to theamorphous metal oxide form at a low temperature by the process of thisinvention, as demonstrated in the preceding paragraph of this example.

EXAMPLE 8 This example shows the undesirable effects of ignition on azirconium oxide felt.

Two identical samples of rayon felt, 6 inches square, were immersed inthe same aqueous solution of 2.17 molar zirconyl chloride. After 200hours the cloth samples were removed from the solution, blotted andcentrifuged to remove excess solution, and were dried. The first samplewas then treated according to the process of this invention under thefollowing conditions: Heated in air at 50 C. per hour to 350 C. and heldat this temperature in air for 4 hours. The temperature was then raisedto 600 C. (in air) over a two hour period and maintained at 600 C. for 2hours.

The second sample was treated as follows: The sample was introduced inair into a furnace already heated to 800 C. and was removed after onehalf hour.

The final product obtained by the process of this invention haddecreased in dimensions to 2%; inch by 2%; inch, an average dimensionalshrinkage of 65 percent, while the other sample was 4% inch by 4% inch,a dimensional shrinkage of only 29 percent. Sample A, the product ofthis invention, was strong and flexible, while Sample B was extremelyweak, crumbly and powdery.

EXAMPLE 9 In this example, cellulosic fibers were immersed in an aqueoussolution of the chlorides of nickel, zinc and iron, the concentration ofions in solution being such that the metals would be adsorbed in thecellulose in the same EXAMPLE 10 Aluminum oxide film Several 2 by 2 inchpieces of cellophane film were immersed in a 2.0 molar aqueous solutionof aluminum chloride for a period of 18 hrs. The cellophane film usedwas of the type generally employed as cigarette package wrapper and waswashed with acetone to remove the moistureproofing lacquer from itssurfaces. After immersion the films were wiped free of unimbibedsolution and dried in a dessicator between two pieces of paper to keepthe film fiat. The film pieces were next heated in air at a rate of 10C./hr. to 365 C. and held at that temperature for 8 hrs. They werefurther heated at 800 C. for 20 hours to remove traces of carbon.

The product film pieces were smooth, had dimensions approximately 0.8 x0.8 inch and thickness of 10-12 microns. The films were flexible,colorless and completely transparent like window glass. The films werecomposed of very poorly crystalline gamma-alumina of 97% purity.

EXAMPLE 11 Mixed aluminum oxide-uranium oxide film Three pieces of thesame cellophane film as used in Example 10 were immersed in an aqueoussolution of 2.0 molar aluminum chloride mixed with 0.08 molar uranylchloride. After immersion the films were wiped free of unimbibedsolution and dried in a dessicator between paper to keep the films flat.The films were next heated in air at a rate of 10 C./hr. to 365 C. andheld for 8 hrs. at that temperature.

The products of this treatment were smooth film pieces with dimensionsapproximately of the starting cellophane film. Thickness of the filmswas 12 microns. The film pieces were substantially smooth, transparentand with a yellow coloration. The films were composed of 27 wt. percentU0 and 72 wt. percent A1 0 and were amorphous as indicated by X-raydiffraction analysis.

EXAMPLE 12 Uranium oxide foam A piece of polyether foam having anopen-cell structure and measuring 3.0 x 3.1 x 1.5 cm. was immersed in anbutyl acetate solution containing cm. uranyl nitrate hexahydrate per100 ml. for a 1 hr. period. The foam was then blotted and allowed todry.

The salt-loaded foam was pyrolyzed in a vacuum tube furnace which hasheated at a rate of 40 C./hr. to 900 C. and held at 900 C. for one hour.The rigid carbonized foam was next held at 750 C. and air admitted tothe furnace slowly over a period of four hours to oxidize the carbonfrom the foam.

The product foam was approximately of the original dimensions of thestarting polyether foam and had the same open-cell structure but ofreduced dimensions.

The foam had 91% open porosity; cell dimensions were in the range of5075 microns. The color of the foam was greenish black, indicative ofthe U 0 composition.

What is claimed is:

1. A process for producing metal oxide fibers, textiles and shapes whichcomprises (1) impregnating a preformed organic polymeric material with acompound of one or more of the metals beryllium, magnesium, calcium,strontium, barium, scandium, yttrium, lanthanim, cerium, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, manganese, iron, cobalt, nickel, copper, zinc, cadmium,aluminum, gallium, silicon, tin, lead, thorium, uranium and plutonium,and (2) heating said impregnated material to a temperature sufficientlyhigh to carbonize and oxidize said organic material and convert saidmetal compound to metal oxide, at least a portion of said heating stepbeing carried out in an oxidizing atmosphere, and said heating stepbeing carried out without ignition of said material.

2. A process in accordance with claim 1 wherein said material isimpregnated by immersion in a solvent solution containing said metalcompound, and wherein excess impregnating solution is removed from saidimpregnated organic material and said impregnated material is driedprior to said carbonization-oxidation step.

3. The process in accordance with claim 2 wherein said organic materialis cellulosic fibrous material.

4. The process in accordance with claim 2 wherein said organic materialis a non-fibrous cellulosic material.

5. The process in accordance with claim 3 wherein saidcarbonization-oxidation step comprises heating said impregnated fibrousmaterial to a temperature between about 350 C. and about 900 C. at arate of not more than C. per hour in a atmosphere containing from aboutto about 25 volume percent oxygen, provided said temperature does notexceed the melting or decomposition temperature of said metal oxide.

6. The process in accordance with claim 3 wherein said fibers aremaintained under tension during said carbonization-oxidation step.

7. The process in accordance with claim 3 wherein saidcarbonization-oxidation step comprises heating said impregnated fibrousmaterial in a non-oxidizing atmosphere to temperatures between about 700C. and 1000 C. in less than one hour followed by heating in said sametemperature range in an atmosphere containing at least 5 volume percentwater vapor until oxidation of said material is substantially complete.

8. The process in accordance with claim 3 wherein saidcarbonization-oxidation step comprises heating said impregnated fibrousmaterial in an atmosphere containing less than about volume percentoxygen to temperatures between about 350 C. and 600 C. in less than onehour and then gradually increasing the oxygen concentration to about 20volume percent or more and maintaining said temperature of 350 C. to 600C. until oxidation of said material is substantially complete.

9. The process in accordance with claim 3 wherein said organic materialis a rayon fiber, said fiber is impregnated by immersion of an aqueoussolution of aluminum chloride, and said impregnated fiber, after removalof excess impregnating solution and drying, is heated in air to 400 C.at a rate of about 100 C. per hour and is 11. The process in accordancewith claim 4 wherein said organic material is a non-fibrous cellulosicfilm impregnated by immersion in an aqueous solution of an aluminumcompound and, after removal of excess solvent and drying, is heated inan oxygen-containing atmosphere to about 365 C. to carbonize and oxidizesaid film and is further heated in an oxygen-containing atmosphere toabout 800 C. to produce a film of aluminum oxide.

12. The process in accordance with claim 11 wherein said aqueoussolution contains both an aluminum compound and a uranium compound andsaid product film contains both aluminum oxide and uranium oxide.

13. The process in accordance with claim 4 wherein said organic materialis a non-fibrous polyether foam impregnated by immersion in an aqueoussolution of a uranium compound and, after removal of excess solvent anddrying, is heated in vacuum to about 900 C. to carbonize said foam andis further heated in an oxygencontaining atmosphere at about 750 C. toproduce a foam of uranium oxide.

References Cited UNITED STATES PATENTS 1,993,778 3/1935 Francis 671003,065,091 11/1962 Russell et al. 106-57 X 3,082,099 3/1963 Beasley eta1. 106-57 X 3,082,103 3/1963 Wainer 106-57 X 3,240,560 3/1966 Spear23142 OTHER REFERENCES BMI-1117, Effect of Ceramic or Metal Additives inHigh-U0 Bodies, 1956, p. 5.

L. DEWAYNE RUTLEDGE, Primary Examiner.

L. A. SEBASTIAN, Assistant Examiner.

